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GCSE Biology: Osmosis in a Potato

Aim

To investigate the effect of placing a piece of potato in a given strength of sugar solution.

Prediction

I predict that, as the solution becomes more concentrated the more the cell will shrink, as the water, of higher concentration inside the potato cells, flows down a concentration gradient into the solution, which has a lower concentration of water molecules. If we say that the percentage change when the potato is placed in the 1M solution is x%. I predict that when the solution is 0.5M the percentage change will be 0.5x%, and when the solution is 0.25M strength I hypothesise the change will be 0.25x%.

Variables Description: Dependant Variables

The dependant variable is plotted on the y-axis. In this experiment this is the change of mass of the potato when it is placed in different strengths of sugar solution.

Variables Description: Independant Variables

The independant variable is plotted on the x axis and is what you choose to change in the experiment. In this experiement it is the strength of the sugar solution.

Variables Description: Constant Variables

These stay the same all the way through the experiment and are the fair test rules. In this experiment that are that the amount of sugar solution used must always me 10.0ml. This is to be placed into the test tube. The solution needs to always be at the same temperature. We will make sure that all experiments are at 20 degrees centigrade before we start the experiment. The size of the potato sample that we place into the solution also needs to be kept constant. A mass of 2.5-2.6g is sensible. The diameter is fixed as the potato cork borer always measures to the exact same measurement. We also needs to make sure that the potato is left in the solution for the same time, 30:00 minutes.

Equipment

Potato

Standard Sugar Solution

Wash Bottle

Test Tubes and Test Tube Rack

Cork Borer

Scalpel

Cutting Board

Scales accurate to 2.d.p

10ml Syringe

Paper towels

Ruler

Stop Watch

Method

Firstly, the cork borer needs to be used to cut out a piece of potato at least 4 cm in length. This allows room for trimming off untidy edges.

On the cutting board the cylindrical piece of potato needs to be cut to 35mm. Using a ruler, measure this as accurately as possible and cut the potato to 35mm in length using a scalpel.

Using the scaled weigh the potato to 2.d.p. and record the weight.

The test tube rack and test tubes need to be assembles. Fill a syringe with exactly 10ml or sugar solution (strength 1M) and empty into a test tube.

Now the timing device needs to be reset and readied. An extra pair of hands to aid in the timing may be usful. Ths potato piece can now be placed into the test tube containing 10mb of 1M sugar solution.

As the potato piece is placed in the test tube, the timer should be started. After 30 minutes the potato piece needs to be removed from the test tube.

Place the potato piece once again on the scales and work out the percentage change.

Repeat this experiment for 0.5, 0.25 and 0.125 concentration solutions of sugar solution.

Results

Mass of Chip at...

Strength of Solution

1

0.5

0.25

0.125

0.0625

0

Start

2.5

2.6

2.5

2.5

2.5

2.6

End

2.8

2.9

2.8

2.9

2.8

2.9

Change

0.3

0.3

0.3

0.4

0.3

0.3

% Change

12

12

12

16

12

12

Concentration of sugar (M)

Start Mass (g)

Finish Mass (g)

% Change

Full

2.5

2.8

12

0.5

2.6

2.9

12

0.25

2.5

2.8

12

0.125

2.5

2.9

16

0.06125

2.5

2.8

12

0

2.6

2.9

12

Conclusion

Our results show that their is no difference between the change in mass of a potato piece weather placed in concentrated sugar solution or a pure water solution. This disagrees with my prediction. My prediction hypothesised that there would be a difference and that half the solution cncentration would result in half the percentage change in mass of potato. There was one anomylous result (0.125, 16) appeared a 16% change, whereas all the other results appeared as a 12% change. No obvious factors can be found for this result, the piece was 2.5g as many of the others were, yet it increased to 2.9 rather than 2.9 as had the others. As the scales were only correct to 1.d.p. it is possible that the amount was very near to the others and yet the measuring was less accurate and the rounding and inaccuracy in measurements caused this result.

Evaluation

The results from this experiment do not agree with my prediction. To get accurate results more precise equipment needs to be used and to be repeated many more times. The test was as fair as it could be made by the imposed constraints. These constraints were factors such as time, and availability of equipment meaning there was not enough time for each experiment to be properly completed. Each potato piece was only got 15 minutes in the solution, whereas 30 would have been more preferable. This could partly explain why the results did not support the prediction. The dependant variable, the change in mass of the potato could not be measure to a satisfactory degree of accuracy because we were using small test tubes we require more sensitive equipment thatn if the experiment were being performed on a larger scale. Unfortunately this was unavailable and so we could only measure the masses to 1.d.p. The solution strength could also have been measured more accurately if syringes had been available instead of measuring cylinders it would have made the solutions more accurate as measuring 0.0625 ml with the available equipment was near impossible. This along with weighing the mass of the potato piece were the two biggest sources of possible errors with the experiment. If the experiment were to be perfomed again, more accurate equipment as describes aboce would be needed. The experiment would need to be tested for ratio's that are equally spreadand with a smaller gap between testing values.